In a GPS system that uses an artificial satellite (GPS satellite) to measure positions of movable bodies, a GPS receiver has a basic function of receiving signals from four or more GPS satellites, computing a receiver's position from received signals, and notifying a user of the receiver's position.
The GPS receiver demodulates a signal from the GPS satellite to obtain the GPS satellite's orbital data. The GPS receiver then derives the receiver's three-dimensional position using simultaneous equations from the GPS satellite's orbit and time information and the received signal's delay time. There are needed four GPS satellites to obtain received signals because an error occurs between the GPS receiver's inside time and the satellite time and it is necessary to remove an effect of the error.
A household GPS receiver receives a spectrum spread signal wave called an L1-band, C/A (Clear and Acquisition) code from the GPS satellite for positioning operations.
The C/A code is a PN (Pseudorandom Noise) sequence code, e.g., a Gold code having a transmission signal rate (chip rate) of 1.023 MHz and a code length of 1023. The C/A code is a BPSK (Binary Phase Shift Keying) modulated signal made of a carrier wave (hereafter referred to as a carrier) having a frequency of 1575.42 MHz by means of a signal spread from 50 bps data. Since the code length is 1023 in this case, the C/A code comprises a repetition of PN sequence codes at a cycle of 1023 chips (i.e., one cycle=one millisecond) as shown in FIG. 28(A).
A PN sequence code of the C/A code is unique to each GPS satellite. The GPS receiver is previously configured to be able to detect which GPS satellite uses which PN sequence code. A navigation message (to be described) is used to indicate from which GPS satellite the GPS receiver can receive signals at a given point and time. During three-dimensional positioning, for example, the GPS receiver receives radio waves available at the point and time from four or more GPS satellites applies an inverse spectrum spread to the radio waves, and performs a positioning operation to find its own position.
As shown in FIG. 28(B), one bit of satellite signal data is transmitted as 20 cycles of PN sequence code, i.e., in units of 20 milliseconds. That is to say, the data transmission rate is 50 bps. The bit set to “1” or “0” inverts one cycle of PN sequence code, i.e., 1023 chips.
As shown in FIG. 28(C), the GPS forms one word in units of 30 bits (600 milliseconds). As shown in FIG. 28(D), one subframe (6 seconds) comprises 10 words. As shown in FIG. 28(E), the first word in a subframe is always prefixed by a preamble as a specified bit pattern even if data is updated. Data is transmitted after the preamble.
One frame (30 seconds) comprises five subframes. A navigation message is transmitted in units of one-frame data. First three subframes in one-frame data constitute satellite-specific information called ephemeris information. This information contains a parameter to find the satellite's orbit and the time to transmit the signal from the satellite.
All GPS satellites have atomic clocks so that each GPS satellite can use the common time information under surveillance of an earth station. The ephemeris in a navigation message from the GPS satellite contains data representing the time, i.e., week number and TOW (time of week).
The week number is 10-bit data representing 0 through 1023 and is incremented every week starting from Jan. 6 (Sunday) in 1980 to be the zeroth week. The TOW is 17-bit data representing 0 through 100800 (=3600×24×7/6) and is incremented every 6 seconds starting from 0:00 a.m. on Sunday.
The GPS receiver can find an absolute time by obtaining the week number and the TOW from the received navigation data. A value smaller than 6 seconds can synchronize with the GPS satellite time in accordance with accuracy of a reference oscillator of the GPS receiver during a process in which the GPS receiver locks on to a signal from the GPS satellite. The PN sequence code of the GPS satellite is generated as synchronized with the atomic clock.
The orbit information in the ephemeris information is updated at an interval of several hours and remains unchanged until updated. The orbit information in the ephemeris information can be stored in the GPS receiver's memory so as to be able to accurately reuse the same information for several hours.
The remaining two subframes in one frame of navigation message constitute so-called almanac information that is commonly transmitted from all satellites. The almanac information requires 25 frames for obtaining the entire information and comprises information about approximate positions of the GPS satellites, information indicative of available GPS satellites, and the like.
The almanac information is updated at least once in six days and remains unchanged until updated. The almanac information takes effect for several months if it is used for the purpose of finding approximate positions of GPS satellites. However, it is desirable for the GPS receiver to appropriately update the almanac information and keep the most recent data.
First, a carrier needs to be removed in order to receive a signal from the GPS satellite and obtain the above-mentioned data. The signal from the GPS satellite is subject to phase synchronization with the C/A code through the use of a PN sequence code (hereafter referred to as a PN code) that is provided in the GPS receiver and is equivalent to the C/A code used for the GPS satellite to be received. In this manner, the signal from the GPS satellite is acquired. Then, an inverse spectrum spread is performed. Bits are detected after the phase synchronization with the C/A code is successful and the inverse spread is performed. It becomes possible to obtain a navigation message containing the time information and the like from the GPS satellite signal.
Signals are acquired from the GPS satellite by means of phase synchronization retrieval for the C/A code. The phase synchronization retrieval detects correlation between the GPS receiver's PN code and the PN code in the signal from the GPS satellite. Both are assumed to be synchronized with each other when the correlation detection result provides a correlation value that is greater than a predetermined value, for example. When both are not assumed to be synchronized with each other, some synchronization technique is used to control the GPS receiver's PN code phase for synchronization with the PN code of the signal from the GPS satellite.
As mentioned above, the signal from the GPS satellite (GPS signal) contains a BPSK-modulated carrier by means of a signal whose data is spread by the PN code (spread code). In order for the GPS receiver to receive the GPS signal, it is necessary to establish synchronization with not only the spread code, but also the carrier and the data. However, it is impossible to establish synchronization with the spread code and the carrier independently.
It is a general practice that the GPS receiver converts the GPS signal's carrier frequency into an intermediate frequency within several megahertz and performs the above-mentioned synchronization detection process using the intermediate frequency signal. A carrier in the intermediate frequency signal contains: a frequency error due to a Doppler shift mainly in accordance with a movement speed of the GPS satellite; and a local oscillator's frequency error generated in the GPS receiver when the GPS signal is converted into the intermediate frequency signal.
Due to these frequency error factors, the intermediate frequency signal contains an unknown carrier frequency. It is necessary to search for the frequency. A synchronization point (synchronization phase) within one cycle of PN code depends on positional relationship between the GPS receiver and the GPS satellite and is therefore unknown. Accordingly, some synchronization technique is needed as mentioned above.
A conventional GPS receiver uses the synchronization technique using a frequency search for the carrier, a sliding correlator, a DLL (Delay Locked Loop), and a Costas loop. This will be further described below.
A clock to drive a generator for the GPS receiver's PN code is generally available by dividing a reference frequency oscillator provided for the GPS receiver. A highly accurate quartz oscillator is used for the reference frequency oscillator. An output from the reference frequency oscillator is used to generate a local oscillation signal for converting a received signal from the GPS satellite into an intermediate frequency signal.
FIG. 29 diagrams the frequency search. The frequency search assumes frequency f1 for a clock signal to drive the generator for the GPS receiver's PN code. The frequency search sequentially shifts the phase synchronization retrieval for the PN code, i.e., the PN code phase by the amount of one chip at a time. The frequency search detects correlation between the GPS received signal and the PN code at the corresponding chip phase. The frequency search detects a peak value for the correlation to detect a phase for establishing the synchronization.
When the clock signal has frequency 1, there may not be available synchronizing phase in all phase retrievals for 1023 chips. In such case, the frequency search changes a division ratio for the reference frequency oscillator, changes the drive clock signal's frequency to frequency f2, and performs a phase retrieval for 1023 chips. This operation is repeated by stepwise changing the frequency of the drive clock signal as shown in FIG. 29. The frequency search operates as mentioned above.
When the frequency search detects a synchronizable frequency of the drive clock signal, the detected clock frequency is used to perform the final PN code phase synchronization. This enables acquisition of satellite signals despite oscillation frequency deviation in the quartz frequency oscillator.
When signals are continuously received from the GPS satellite for positioning according to a conventional manner, the synchronization for the carrier and the PN code is acquired and the acquired synchronization is held. For this reason, it is necessary to continuously operate circuits in the GPS receiver, especially circuits for the DLL and the Costas loop.
Further, the positioning operation requires a distance (range) between a receiver P and a GPS satellite ST1 or ST2 as shown in FIG. 30. The range can be sufficiently obtained at a specified cycle such as every 0.5, 1, or 2 seconds, for example. Normally, the range is measured at a relatively short interval such as 0.1 seconds (100 milliseconds) as shown in FIG. 31, for example, so as to always accurately obtain the range.
Consequently, the GPS receiver is always in full operation, increasing power consumption for the GPS receiver. The GPS receiver is mounted on a movable body or is carried by a user for operation. It is important to accurately operate the GPS receiver using the power from a battery for as long a time as possible.
In order to decrease power consumption of the GPS receiver, an intermittent operation is considered to repeatedly turn on or off the GPS receiver.
However, intermittently operating the GPS receiver may sacrifice the GPS receiver performance such as positioning sensitivity or positioning speed compared to the state of always activating the power. That is to say, intermittently operating the GPS receiver may disable accurate positioning, detect just an approximate position of the GPS receiver, or consume time to detect an accurate position.
The above-mentioned problems become particularly serious when a user manually performs an intermittent operation or when an intermittent operation is performed at a cycle that gives no consideration to an acquisition cycle of range data or an output cycle of positioning information indicative of the GPS receiver's current position.
The present invention has been made in consideration of the foregoing. It is therefore an object of the present invention to provide a GPS receiver and a GPS signal reception method capable of periodically obtaining data within a range needed for range computation range data without degrading the performance such as positioning sensitivity and positioning speed and conserving energy consumption.